The only real way in which these things are different is that scientific management is one way to achieve rationalization. The whole purpose of scientific management is to rationalize the production processes of a firm. As the link below tells us, scientific management can be defined as the
complete rationalization of all processes involved in the production of a product.
When we talk about rationalizing the processes involved in production, we are talking about making those processes more efficient. To rationalize the processes means to make them less improvised and ad hoc. It means to take the processes and arrange them in a more logical way so as to make them go quicker. Scientific management is simply one way to accomplish this. Therefore, the only difference is that rationalization is the end and scientific management is a means to that end.
The question asks how rationalization is different from scientific management. The term “scientific management” here refers to the principles of business management, most commonly attributed to Frederick Taylor, which guided the development of mass production techniques in the early 20th century. Henry Ford's work in applying these principles to automobile manufacture is also instructive.
The underlying concept is that work, like all other aspects of human experience, could be analyzed and improved upon through application of the scientific method. To recap, this method involves observation, establishment of hypotheses about cause and effect relationships, and experiments with control of key variables to prove or disprove these hypotheses. Most importantly, it is based on the belief (faith) that all of the relevant causal relationships and their interactions can be understood. Applying this to business management, the idea is to analyze (break apart) every step in a production process, determine the most efficient way to perform it (including the decision regarding the amount of capital versus labor to apply to it), then review the results and revise the process accordingly. The most important results are expected to be predictability and reproducibility of results; “I know exactly what I will get” and “I get the same outcome every time”.
In practice, this had the effect of the development of the assembly line style of production. In particular, human work transitioned from an artisan approach to one in which workers were viewed as “cogs in the machine.” The former involved workers with high levels of individual skill, acquired through an apprenticeship process, performing multiple and varied tasks (e.g. the metal worker who crafted the frame of an early automobile). Early automobiles were manufactured under this approach by teams of craftsmen who created a car like a fine piece of furniture. By contrast, mass production techniques involved a different person dedicated to each step in a process; for example, one person welding the front member of a frame to the side rails, then sending it down to the next person to weld the back member. Each person only needed to understand a single, relatively simple task, which they performed repetitively throughout the work day. The result was that, even though more individuals were involved in the production of a single car, their ability to focus on one task and repeat it quickly led to much higher output per person, lowering costs and improving profits. As a side note, there is a substantial body of literature analyzing the sociological and psychological impacts of this on human well-being.
The act of designing and implementing these production techniques can be broadly viewed as “rationalization.” Therefore, rationalization can be viewed as the implementation phase of the scientific management technique, i.e. the culmination of the entire technique which started with observation and analysis. It is the point at which the technique transitions from “research” to “development,” from science to engineering.